1. Field of Invention
The present invention relates to a radiation image storage panel employed in a radiation image recording and reproducing method utilizing a stimulable phosphor.
2. Description of Prior Art
As a method replacing a conventional radiography, a radiation image recording and reproducing method utilizing a stimulable phosphor as described, for instance, in U.S. Pat. No. 4,239,968, is known. In this method, a radiation image storage panel comprising a stimulable phosphor (i.e., stimulable phosphor sheet) is employed, and the method involves the steps of causing the stimulable phosphor of the panel to absorb radiation energy having passed through an object or having radiated from an object; sequentially exciting the stimulable phosphor with an electromagnetic wave such as visible light or infrared rays (hereinafter referred to as "stimulating rays") to release the radiation energy stored in the phosphor as light emission (stimulated emission); photoelectrically detecting the emitted light to obtain electric signals; and reproducing the radiation image of the object as a visible image from the electric signals.
In the radiation image recording and reproducing method, a radiation image is obtainable with a sufficient amount of information by applying a radiation to an object at a considerably smaller dose, as compared with that of the conventional radiography. Accordingly, this method is of great value especially when the method is used for medical diagnosis.
The radiation image storage panel employed in the radiation image recording and reproducing method basically comprises a support and a stimulable phosphor layer provided thereon. Further, a transparent film is generally provided on the free surface of the phosphor layer (surface not facing the support) to keep the phosphor layer from chemical deterioration and physical shock.
The stimulable phosphor layer generally comprises a binder and stimulable phosphor particles dispersed therein. However, the stimulable phosphor layer can be in the form of a layer made by vapor-deposition or a sintered layer. The stimulable phosphor emits light (gives stimulated emission) when excited with an electromagnetic wave (i.e., stimulating rays) such as visible light or infrared rays after having been exposed to a radiation such as X-rays. Accordingly, the radiation having passed through an object or radiated from an object is absorbed by the phosphor layer of the panel in proportion to the applied radiation dose, and a radiation image of the object is produced in the panel in the form of a radiation energy-stored image. The radiation energy-stored image can be released as stimulated emission by sequentially irradiating (scanning) the panel with stimulating rays. The stimulated emission is then photoelectrically detected to give electric signals, so as to reproduce a visible image from the electric signals.
The radiation image recording and reproducing method is very advantageous for obtaining a visible image as described above, and the storage panel used in the method is desired to have high sensitivity and provide an image of high quality (high sharpness, high graininess, etc.).
In performing the radiation image recording and reproducing method, the radiation image storage panel is repeatedly used in a cyclic procedure comprising the steps of: exposing the panel to a radiation (recording radiation image thereon), irradiating the panel with stimulating rays (reading out the recorded radiation image therefrom) and irradiating the panel with a light for erasure (erasing the remaining radiation image from the panel). The panel is transferred from a step to the subsequent step in a transfer system in such a manner that the panel is sandwiched between transferring members (e.g., rolls and endless belt) of the system, and piled on other panel to be stored after one cycle is completed.
The repeated use of the storage panel comprising transferring and piling causes physical contacts such as a friction between the surface of the panel (surface of the phosphor layer or surface of the protective film) and a surface of other panel (surface of the support), friction between edges of the panel and a surface of other panel, and a friction between the panel and transferring members (e.g., roll and belt).
As a support material of the radiation image storage panel, desirably employed are plastic films (i.e., polymer films) such as a polyethylene terephthalate film and one of various papers (coated or uncoated) from the viewpoint of flexibility required in the transferring procedure of the panel.
However, the panel having a support made of a polymer material or a paper is apt to be electrostatically charged on its surface owing to the repeated physical contact encountering in the transferring procedure. In more detail, the surface (front surface) of the panel is apt to be negatively charged, and other surface (back surface) is apt to be positively charged. This static electrification causes various problems in the practical operation of the radiation image recording and reproducing method.
For example, when the surface of the panel is electrostatically charged, the surface of the panel easily adheres to a surface of other panel and thus adhering panels hardly separate from each other, for instance, in the vertical direction against the panel surface. In that case, the panels are transferred together in the form of a composite, from the piling position into the transfer system, whereby the subsequent procedure cannot be normally conducted.
In addition, the read-out procedure of the panel is generally carried out by irradiating the panel with stimulating rays from the phosphor layer-side surface of the panel, and in this procedure, the charged surface of the panel is likely to be deposited with dust in air, so that the stimulating rays are scattered on the dust deposited on the charged surface and quality of the resulting image lowers. Moreover, the storage panel decreases in the sensitivity or the resulting image provided by the panel suffers noise such as static mark when discharge takes place, and unfavorable shock is sometimes given to the operator because of the spontaneous discharge from the panel.
Japanese Patent Provisional Publication No. 62(1987)-87900 discloses a radiation image storage panel having a antistatic layer on its back side (that is, on a surface of a support on the side not facing the phosphor layer). The antistatic layer is made of an electroconductive material such as metal film, powdery metal oxide, carbon black or electroconductive organic material and shows a specific surface resistivity of not higher than 10.sup.11 ohm.
Japanese Patent Provisional Publication No. 62(1987)-174700 discloses a radiation image storage panel having a antistatic layer between the support and the phosphor layer. The antistatic layer is made of an electroconductive material such as powdery metal oxide, carbon black or an electroconductive organic material and shows a specific surface resistivity of not higher than 10.sup.12 ohm.
Japanese Patent Provisional Publication No. 63(1988)-167298 discloses a radiation image storage panel containing K.sub.2 O.nTiO.sub.2 whisker at least in a portion thereof.